As the demand for broadband multimedia communication services such as the Internet and video distributions has explosively increased, high-density wavelength multiplexing optical fiber communication systems having longer distances, larger capacities, and higher reliability have been increasingly introduced in trunk systems and metro-systems (intercity systems). Further, optical fiber access services are also becoming rapidly widespread in subscriber systems. For such communication systems using optical fibers, it is important to reduce the cost required for laying (i.e., constructing) optical fibers serving as optical transmission paths and increase the transmission band use efficiency per optical fiber.
Therefore, a wavelength multiplexing technique in which a plurality of optical signals having different wavelengths are multiplexed and transmitted in the multiplexed state has been widely used. Further, since the increasing traffic cannot be handled by using the wavelength multiplexing technique alone in recent years, the demand for increasing the transmission capacity per wavelength channel has grown even further. For increasing the transmission capacity per wavelength channel, it is advantageous to employ a multi-value optical modulation signal technique whose optical modulation spectrum bandwidth is narrower than that of an ordinary binary optical intensity modulation technique in view of the spectrum use efficiency and the tolerance to the wavelength dispersion and the polarized-wave dispersion of the optical fibers.
For example, optical communication systems using a digital coherent technique, which has started to be commercially used in recent years, uses QPSK (Quadrature Phase Shift Keying) signals. Further, to increase the capacity even further, optical communication systems and the like using larger multi-values such as 16-QAM (Quadrature Amplitude Modulation) have been studied. Further, to increase the capacity per wavelength channel, it is necessary to improve the symbol frequency of data. The bands of components are an important factor for improving the symbol frequency of data.
In the case of optical components commonly used in optical fiber communication systems, for example, since each of such components itself has a capacitance in an actual optical modulator, its response speed is finite and hence does not have an ideal response characteristic. Therefore, since the transient response waveform of a phase modulation is determined based on the band of the drive device that drives the optical modulator, i.e., the transient response speed (rising and falling times) of an electric signal and the band characteristic of the optical modulator itself, there has been a limit on the increase in the speed exceeding 10 Gb/s.
To solve such problems, a related art technique is proposed in Patent Literature 1. FIGS. 14A and 14B show a concept of a phase modulation section of the related art. FIG. 14A shows its configuration diagram, and FIGS. 14B(a) and 14B(b) show waveforms showing a phase modulation state. In the related art, a first optical modulator 101 and a second optical modulator 102 are provided on an optical waveguide. Further, the phase of light passing through the optical waveguide 103 is modulated by applying voltages to these optical modulators.
In this related art, a desired electric input signal DIN is applied to the first optical modulator 101 and a signal DINB, which is delayed from the electric input signal DIN by a timing T and has a polarity reverse of that of the electric input signal DIN, is applied to the second optical modulator 102. It should be noted that the amplitude of the signal DINB is smaller than that of the electric input signal DIN and appropriately chosen. By doing so, as shown in FIG. 14B(a), it is possible to form overshoots and undershoots immediately after the rising/falling edges of the waveform with respect to the original phase modulation waveform. FIG. 14B(a) shows an ideal rectangular waveform having an infinite response speed for the sake of an easier explanation. However, in an actual optical modulator, since the bands of the drive device and the optical modulator are finite, the waveform becomes the one shown in FIG. 14B(b).
As described above, the related art realizes a high-speed optical phase modulation by appropriately setting the delay amount τ and the amplitude of the signal DINB by using the device configuration and thereby performing a waveform shaping process in which the overshoot and undershoot amounts can be appropriately controlled even when the operating speeds of the drive electric device and the optical modulator themselves are not very high.
However, although the above-described optical phase modulator can control overshoots/undershoots immediately after the rising and falling edges of the phase modulation waveform, it cannot control the waveform immediately before the rising and falling edges of the waveform. That is, there is a problem that when the phase modulation waveform is observed as an eye pattern, the waveform at the last part of the eye pattern cannot be shaped, though the waveform at the starting part of the eye pattern can be shaped.
Further, a pulse waveform that is shaped by a device using the above-described technique becomes a left-right asymmetric waveform. This prevents the frequency characteristics of the drive device and the optical modulator, which are affected by various factors, from being perfectly corrected, thus making it impossible to obtain a desired waveform. Further, the related art generates overshoots/undershoots by subtracting the polarity-reversed signal DINB from the original waveform. This is a de-emphasis technique, causing a problem that the amplitude is lowered.
Therefore, to obtain a desired phase change amount (a desired amplitude of the waveform), it is necessary to increase the amplitude of the DIN signal in advance by an amount corresponding to the phase change amount caused by the signal DINB. This leads to a significant increase in the electric power consumed by the drive device. Further, the related art has a problem that it cannot cope with the waveform shaping of multi-value modulations, which are expected to become main-stream in the future.